A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to accurately apply a desired pattern onto a target portion of a substrate, the reticle should be aligned with respect to the substrate. Therefore, according to the prior art, the relative position of the reticle with respect to the substrate is set accurately, by measuring and adjusting the relative position. Alignment of the patterning device with respect to the substrate may, according to the state of the art, be done using two alignment actions.
In the first action the substrate is aligned with respect to the substrate stage carrying the substrate, while in the second action the reticle is aligned with respect to the substrate stage. As a result of these two actions, the reticle is aligned with respect to the substrate, as desired.
In case a single stage machine is used, the first and second action are carried out at the exposure position. In case a dual stage machine is used, the first action may be carried out at a first position, remote from the exposure position. Then, the substrate stage with the substrate positioned on it is transported to the second position, i.e. the exposure position, where the second action is performed.
The first action may be carried out with two sensor assemblies. A first sensor assembly comprises an alignment sensor and measures the relative position of the substrate with respect to the substrate stage in X, Y and Rz directions, where the XY plane is defined as the plane that is substantially parallel with the surface of the substrate, the X- and Y-direction being substantially perpendicular with respect to each other. The Z-direction is substantially perpendicular with respect to the X- and Y-directions, so Rz represents a rotation in the XY plane, about the Z-direction. A more detailed description about this sensor is for instance provided in U.S. Pat. No. 6,297,876. A second sensor assembly, usually referred to as the level sensor, measures the height of the substrate surface in dependence on locations on the substrate to be exposed, creating a height map based on the determined heights, and also determines the rotations about the X and Y axes: Rx, Ry.
Next, in the second action, the reticle is aligned with respect to the substrate stage. This may be done with an image sensor, such as a transmission image sensor, as will be known to a person skilled in the art. A transmission image sensor measurement is performed by imaging a first alignment pattern (mask alignment mark) provided on the reticle or on the reticle stage carrying the reticle, through the projection system (lens) onto one or more plates (i.e. the transmission image sensor plate) provided at or in the substrate stage. The transmission image sensor plate comprises a second alignment pattern. The alignment patterns may include a number of isolated lines. Inside the substrate stage, behind the second alignment pattern in the transmission image sensor plate, a light sensitive detector is provided, e.g. a diode, that measures the light intensity of the imaged first alignment pattern. When the projected image (i.e. the aerial image) of the first alignment pattern exactly matches the second alignment pattern, the sensor measures a maximum intensity. The substrate stage is now moved in the X- and Y-directions on different Z-levels, while the sensor measures the intensity. Therefore, the transmission image sensor is actually an aerial image sensor, in which multiple scanning slits probe the aerial image of isolated lines. Based on these measurements, an optimal relative position of the substrate stage can be determined. Below a typical transmission image sensor will be explained in more detail.
As mentioned above, in the first action, the alignment sensor measures the position of the substrate with respect to the substrate stage carrying the substrate. The alignment sensor also measures the XY position of the transmission image sensor plate, more specifically the position of a fiducial mark on the transmission image sensor plate, while the level sensor, in combination with a further sensor (Z-interferometer), measures the Z-position thereof Based on the position of the substrate with respect to the substrate stage and the position of the transmission image sensor with respect to the substrate stage, the position of the substrate relative to the transmission image sensor can be determined.
As mentioned above as well, in the second action the reticle is aligned with respect to the substrate stage. The position of the aerial image may be measured by the transmission image sensor and this gives the position of the aerial image with respect to the transmission image sensor. The information from both actions may be combined to calculate the optimal position of the substrate stage (and possibly to determine the lens corrections as well) for the best match of the aerial image and the substrate.
Furthermore it has been proposed to immerse the substrate in the lithographic apparatus in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. This enables imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid relative to the wavelength in air. The effect of the liquid may also be regarded as increasing the effective numerical aperture NA of the system and also increasing the depth of focus.
One of the known drawbacks of the immersion type lithographic apparatus is the presence of liquid, for instance water, usually in the form of droplets, at the surface of the substrate and the surrounding structure, for instance at the mirror block and at the sensors (for instance the transmission image sensor) provided in the mirror block. More specifically, the liquid may be present on the upper surface of the transmission image sensor plate, which typically is made of quartz material. More specifically, most of the surface of the transmission image sensor plate is coated by Cr and TiN on top of Cr. Cr and TiN are hydrophilic materials. Consequently, the liquid will take the form of a large number of liquid droplets on the plate. The presence of the liquid on top of the transmission image sensor plate appears to be unavoidable, especially for high throughputs, i.e. for high-speed immersion type lithographic apparatus. While a large portion of the liquid droplets evaporate relatively fast (for instance in about 15-25 seconds), they can have a significant (and negative) influence on the substrate production process as a whole. A liquid drop may, for instance, act as a lens for the alignment and level sensors, thereby disturbing their proper functioning and possibly decreasing their accuracy. Furthermore, the drying liquid may locally reduce the temperature of the sensor(s). As a result distances between marks and their height change due to the thermal expansion and increased tensions in the sensor structure, thereby disturbing the measurement process.
To avoid these disadvantages the sensor surface may be covered with a water repellent coating. However, this may prove problematic because the aerial image can be mapped incorrectly due to its location in two media with different refractive indices. Furthermore there are complexities of modeling of an aerial image located in two (or more) different media. This is important for the fit algorithm's used in practice. Furthermore, if the coating is relatively thin, it can be easily destroyed by radiation, for instance UV radiation, and/or corrupted by hard dust particles. Another disadvantage is that it will repeat the shape of the underlying grating, giving the coating a “rough” upper surface.
More specifically, marks provided at the front sensor surface may have topography even after deposition of thick layer of hydrophobic coating. The topography is caused by the large grooves of the alignment marks. The topography can be a source of contamination. It can easily trap the water droplets or particles like resist flakes in the water. Furthermore, cleaning the sensor surface may be difficult due to the presence of the grooves. One may try to reduce the topography by sub-segmentation of the alignment marks with a view to planarize the marks. However, sub-segmentation requires higher manufacturing cost of the sensor and might lead to loss of alignment signal.